The present invention relates to a brake pressure control device and a method for activating and deactivating a hydraulic brake booster.
From British Patent No. 2,281,364, describes a brake pressure control device for a hydraulic dual-circuit braking system having a pneumatic brake booster. For each braking circuit, an arrangement of valves is provided as well as a return pump. Furthermore, an electronic control unit is present, which, from processing sensor signals containing information on the manner of the activation of the brake pedal, generates signals for driving switch-over valves, pre-charge control valves, intake and exhaust valves, and the return pumps. A pneumatic brake booster described in this British patent includes two chambers, separated from each other by a valve, of which one is operated as a low-pressure chamber and the other as a working or drive chamber. A modulation valve is provided, using which the working chamber of the brake booster is ventilated. The purpose of the brake pressure control device is to assure the highest possible values for the vehicle deceleration in response to an automatically controlled hard stop. An automatically controlled hard stop of this type is known as a xe2x80x9cbraking assistant.xe2x80x9d The braking pressures necessary for the automatic hard stop are generated with the assistance of the valve arrangement and the return pumps, i.e., through the appropriate driving of the hydraulic unit.
British Patent No. 2,281,364 also describe, a target braking, i.e., a braking process in normal traffic circumstances monitored by the driver, occurs without the support of the hydraulic unit. However, in this case, which nevertheless arises predominantly in normal traffic circumstances, the power boost is accomplished by the pneumatic brake booster alone. The latter must therefore be designed to attain the maximum required braking pressures. In a pneumatic brake booster of this type, the greatest disadvantage is its size, which is determined by the braking pressures to be attained. It is also disadvantageous in this arrangement that the solenoid valves of the valve arrangement, in particular the switch-over and the pre-charge control valves, have to be designed for the comparatively high master cylinder pressures that are produced using a pneumatic brake booster of this type. In contemporary brake boosters, these pressures are in the order of magnitude of up to 250 bar.
German Patent No. 195 01 760 describes, a method and a device for controlling an ABS-ASR system, using which it is possible to replace the vacuum brake booster entirely or partially with a hydraulic brake booster. The hydraulic power boost is accomplished by a controlled driving of the valve arrangement and the return pumps. However, a complete replacement of the pneumatic brake booster has the consequence that the return pumps of the braking system must be set in operation in every braking process in which an increase or boosted braking pressure is to be generated in the wheel cylinders. This has the disadvantage that the return operation can lead to pedal pulsations, which, in particular at lower pedal forces, can be perceived as disturbing. As a further point, a braking system based exclusively on a hydraulic power boost offers less redundancy in the event of a failure or defect in the hydraulic unit.
Unpublished German Patent Application No. 197 56 080 describes, a brake pressure control unit which is improved with regard to contradictory requirements, the greatest possible comfort combined with the greatest possible reliability and smallest possible space. For this purpose, the device described there has a pneumatic brake booster, which is designed such that it reaches its modulation point, i.e., the point at which the brake pressure support can no longer be increased using the pneumatic brake booster, at a master cylinder pressure of more than zero and less than 50 bar. In this context, an essential difference in the functioning of the xe2x80x9cbraking assistantxe2x80x9d lies in the fact that a substantially proportional dependency exists between the control pressures and the braking pressures in the wheel braking cylinders over a broad operational range of the braking system, i.e., as far as possible all of the pedal positions of the brake pedal and therefore for virtually all of the control pressures input by a driver of the vehicle.
It is also known from this application to provide means for displaying the arrival at the modulation point of the pneumatic brake booster. These means can be designed so that they determine a differential pressure between the working chamber of a pneumatic brake booster and an external ambient pressure, and they, in the event that a threshold value is not reached, generate an identifier representing the modulation point. A method of this type yields very good results in quasi static braking processes. However, it has proven to be the case that in response to a rapid application of the brakes, an activation of the hydraulic boost can under certain circumstances occur relatively late, so that a significant resistance threshold is noticeable during braking.
An objective of the present invention is, inter alia, to make possible a braking process which takes optimal account of the requirements of pedal comfort, capacity for quantitative regulation of the braking force, and reliability even in response to different brake application dynamics (pedal actuation speed).
This object is achieved by providing a brake pressure control unit and a method for activating and/or deactivating a hydraulic brake booster in according with the present invention. In addition, further advantages are yielded.
The brake pressure control unit according to the present invention and the method according to the present invention make possible an optimal driving of a hydraulic brake booster, so that in particular in response to a rapid application of the brakes, an activation of the hydraulic brake booster takes place promptly, so that losses of comfort or of reliability can be effectively avoided. By taking into account the time-related derivation of the master cylinder pressure and the differential pressure between the pressure in the working chamber or the vacuum chamber of the brake booster and the ambient pressure, an optimal application of a hydraulic brake booster using a hydraulic unit can be achieved, without making concessions in driving comfort. The control unit according to the present invention, in an optimal manner, satisfies the requirements of pedal comfort, capacity for quantitative regulation of the braking force, and reliability, and also makes it possible that, in contrast to an evaluation of the differential pressure between the working chamber of a brake booster and an external ambient pressure, the differential pressure between the low-pressure chamber of a brake booster and an external ambient pressure is evaluated. In the event that the differential pressure between the low-pressure chamber of the brake booster and an external ambient pressure is measured, but the pressure in the working chamber of the brake booster is not measured by a measuring signal during the brake actuation, the pressure in the working chamber of the brake booster can be calculated, for example, taking into account the differential pressure between the low-pressure chamber and the ambient pressure, the master cylinder pressure, the time-related derivations of the master cylinder pressure, the duration of braking, and the time intervals of the braking duration.
According to the present invention, it may be advantageous to determine the drive trigger point, at which the hydraulic unit is activated and/or deactivated, by taking into account the differential pressure between the pressure in the working chamber or the pressure in the low-pressure chamber and the ambient pressure when the brake pedal is unactivated. In particular, it is relatively easy to take into account a differential pressure of this type when the brake pedal is unactivated and leads to good results.
It may also be advantageuos to determine the drive trigger point at which the hydraulic unit is activated and/or deactivated by taking into account the time-related derivation of the master cylinder pressure, the differential pressure between the pressure in the working chamber and the ambient pressure when the brake pedal is unactivated, as well as the time-related derivation of the differential pressure between the pressure in the working chamber and the ambient pressure. By taking into account the parameters cited, it is possible in a simple manner to achieve good measuring results. According to a further advantageous embodiment, it is also possible that the drive trigger point at which the hydraulic unit is activated and/or deactivated is also determined by taking account of the time-related derivation of the master braking cylinder pressure and of the differential pressure between the pressure in the working chamber and the ambient pressure when the brake pedal is unactivated, while taking account of a calculated time-related derivation of the differential pressure between the pressure in the working chamber and the ambient pressure, from knowledge of the vacuum booster characteristic, the time-related derivation, the braking duration, and the differential pressure.
According to an advantageous embodiment of the brake pressure control unit according to the present invention and of the method according to the present invention, the drive trigger point for activating the hydraulic unit is determined using the differential pressure between the pressure in the working chamber and the ambient or atmospheric pressure as a master cylinder pressure threshold value according to a formula (1) of the form
pHZxe2x80x94threshold=pHZ1xe2x88x92k1*(d pHZ/dt)+k2*xcex94pB0xe2x88x92k3*f(xcex94pB, dxcex94pB/dt);
and using the differential pressure between the pressure in the vacuum chamber and the ambient pressure as a master cylinder pressure threshold value according to a formula (2) of the form
pHZxe2x80x94threshold=pHZ1xe2x88x92k1*(d pHZ/dt)+k2*xcex94pB0xe2x88x92k3*f(pHZ, d pHZ/dt, tbrake, xcex94tbrake);
pHZ1, k1, k2, and k3 being system constants to be determined and f(xcex94pB, dxcex94pB/dt), f(pHZ, d pHZ/dt, tbrake, xcex94tbrake) being system functions. The constants or parameters to be determined as well as the system functions are advantageously ascertained in the context of driving tests and/or simulations.
It is clear on the basis of the formulas provided that the hydraulic boost is a function of the brake application dynamic and of the potential capacity of the pneumatic brake booster (booster capacity). In response to a rapid application of the brakes, the activation of the hydraulic booster takes place earlier than in response to a quasi static brake pedal actuation. The dynamic terms of the equation therefore have a negative sign. By using the method according to the present invention, it is possible to do without stipulating a concrete modulation point of a pneumatic brake booster as well as sensing the pressure regulating valve stroke.
In response to a quasi static brake pedal actuation, the drive trigger point of the brake booster can be reached in the limiting case. This is related to the fact that the differential pressure between the working chamber pressure and the ambient pressure is a function of the rod path and of the actuation speed of the brake pedal.
It is further advantageous that the drive trigger point of the master cylinder pressure threshold value is determined for the de-activation according to a formula (3) of the form
pHZxe2x80x94threshold=pHZ2xe2x88x92k4*(d pHZ/dt)+k5*xcex94pB0
pHZ2, k4, k5 again representing system constants. The determination of constants k4, k5 is advantageously made again using driving tests or simulations.
The master cylinder pressure threshold value for the activation of the hydraulic unit advantageously corresponds to that for the deactivation of the hydraulic unit.